This invention relates in general to novel bismaleimide compositions. In one aspect, this invention relates to novel aromatic maleimide compounds such as alpha, alpha''-bis(4-maleimidophenyl)-meta-diisopropylbenzene and alpha, alpha'-bis(4-maleimidophenyl)-para-diisopropylbenzene, and to related maleimide-isomaleimide compounds. The invention further relates to the use of maleimide compositions as resin precursors.
Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aircraft, automotive, and sporting goods applications. Typically they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix.
Most advanced composites are fabricated from prepreg, a ready-to-mold sheet of reinforcement impregnated with uncured or partially cured resin. Resin systems containing an epoxide resin and aromatic amine hardener are often used in prepreg. However, most epoxy formulations absorb moisture which reduces their high temperature properties. As a result they are not suitable for use at 350.degree. F. or greater in a moisture saturate condition.
Prepreg resins for use at 350.degree. F. have been based on aromatic bismaleimides, often in combination with additional, usually liquid diluents which may contain other reactive groups such as amines, epoxides, cyanates or the like.
A variety of aromatic bismaleimides are presently available which comprise one, two or four aromatic rings in the structure. In the parent application U.S. Pat. No. 761,432, filed Aug. 2, 1985, the entire teachings of which are incorporated herein by reference, there is disclosed a group of bismaleimides containing three aromatic rings in the molecule. Bismaleimides containing one or two aromatic rings tend to give formulations which are characterized by high Tg, together with low toughness and high water absorption. Equivalent formulations in which the aromatic nucleus has four or more aromatic rings may have better water absorption and toughness properties, but exhibit low Tg values. However, as is disclosed in the parent application, novel bismaleimides containing three aromatic rings in the structure are useful in preparing matrix resin formulations with an improved balance of properties.
Also disclosed in the art, for example in U.S. Pat. No. 2,980,694, are aromatic isomaleimides, i.e., compounds having the structure: ##STR1##
Although such compounds have been isolated, they generally are unstable and readily rearrange either thermally or chemically to the corresponding maleimide structure. For that reason, their preparation from the corresponding amide acids is usually accomplished using reaction conditions selected to avoid isomerizing the resulting isomaleimide structure.